In many grinding operations, grinding tool porosity, particularly porosity of a permeable or an interconnected nature, improves efficiency of the grinding operation and quality of the work-piece being ground. In particular, the volume percent of interconnected porosity or fluid permeability has been found to be a significant determinant of grinding performance of abrasive tools. The interconnected porosity allows removal of grinding waste (swarf) and passage of cooling fluid within the wheel during grinding. Also, the interconnected porosity provides access to grinding fluids such as lubricants between the moving abrasive grains and workpiece surface. These features are particularly important in deep cut and modern precision processes (e.g., creepfeed grinding) for high efficiency grinding where a large amount of material is removed in one deep grinding pass without sacrificing the accuracy of the workpiece dimension.
Examples of such abrasive tools having a very open and permeable structure include abrasive tools utilizing elongated or fiber-like abrasive grains. U.S. Pat. Nos. 5,738,696 and 5,738,697 disclose methods for making bonded abrasives utilizing elongated or fiber-like abrasive grains having an aspect ratio of at least about 5:1. One example of such abrasive tools employing filamentary abrasive grains is currently commercially available under the ALTOS™ trademark from Saint-Gobain Abrasives in Worcester, Mass.
ALTOS™ abrasive tools employ sintered sol gel alumina ceramic grains (Saint-Gobain Abrasives in Worcester, Mass.) with an average aspect ratio of about 7.5:1, such as Norton® TG2 or TGX Abrasives (hereinafter “TG2”), as a filamentary abrasive grain. ALTOS™ abrasive tools are highly porous and permeable grinding tools that have been shown to have high metal removal rates, improved form holding and long wheel life, along with a greatly reduced risk of metallurgical damage (see, for example, Norton Company Technical Service Bulletin, June 2002, “Altos High Performance Ceramic Aluminum Oxide Grinding Wheels”). ALTOS™ abrasive tools use abrasive grains that include only the filamentary abrasive grain, e.g., TG2 grain, to achieve maximum structural openness according to fiber-fiber packing theories (see, for example, U.S. Pat. Nos. 5,738,696 and 5,738,697, the entire contents of which are hereby incorporated by reference). It is generally believed that blending TG2 grain with a significant quantity of other non-filamentary, such as sphere-like, grains would either compromise the structural openness or compromise surface finish of a metal workpiece. However, TG2 grains, although very durable, are not friable enough for certain applications and TG2 grain is more costly to manufacture than most blocky or sphere shaped grains.
Therefore, there is a need to develop a more friable, more cost effective abrasive tool having performance characteristics similar to the performance of abrasive tools employing filamentary abrasive grains, such as ALTOS™ abrasive tools.